nixos/environment: set GTK_DATA_PREFIX

Many desktop environment modules are already setting
this so it already makes sense to just do this globally.

nixos/modules/services/editors/emacs.nix

@@ -95,13 +95,7 @@ in {
 
     environment.systemPackages = [ cfg.package editorScript desktopApplicationFile ];
 
-    environment.variables = {
-      # This is required so that GTK applications launched from Emacs
-      # get properly themed:
-      GTK_DATA_PREFIX = "${config.system.path}";
-    } // (if cfg.defaultEditor then {
-        EDITOR = mkOverride 900 "${editorScript}/bin/emacseditor";
-      } else {});
+    environment.variables.EDITOR = mkIf cfg.defaultEditor (mkOverride 900 "${editorScript}/bin/emacseditor");
   };
 
   meta.doc = ./emacs.xml;